1. Field of the Invention
This invention pertains to an improved olefin extrusion composition, a process for extrusion coating a substrate or article, and a process for making an extrusion cast film. More particularly, this invention pertains to extrusion coating and extrusion cast film compositions with higher drawdown rates, lower neck-in and higher resistance to draw resonance. The extrusion compositions contain at least one substantially linear ethylene polymer as the thermoplastic component and may beneficially include at least one high-pressure ethylene polymer and/or at least one linear ethylene polymer.
2. Technical Background
It is known that low density polyethylene (LDPE) made by high-pressure polymerization of ethylene with free-radical initiators as well as heterogeneous linear low density polyethylene (LLDPE) and ultra low density polyethylene (ULDPE) made by the copolymerization of ethylene and .alpha.-olefins with Ziegler coordination (transition metal) catalysts at low to medium pressures can be used to extrusion coat substrates such as paper board as well as used to prepare extrusion cast film for applications such as disposable diapers and food packaging. However, although LDPE generally exhibits excellent extrusion processability and high extrusion drawdown rates, LDPE extrusion coatings and cast films lack sufficient abuse resistance and toughness for many applications. Conversely, while LLDPE and ULDPE extrusion coatings and cast films offer improved abuse resistance and toughness properties, these ethylene .alpha.-olefin polymers can not be extruded or drawn at high take-off rates and they are known to exhibit relatively poor extrusion processability.
The ultimate extrusion drawdown rate of known linear ethylene polymers is limited (at otherwise practicable extrusion line speeds) by the onset of a melt flow instability phenomena known as draw resonance rather than being limited by melt tension breaks due to "strain hardening" which occurs at higher line speeds and is typical for LDPE and other highly branched high pressure ethylene polymers such as, for example, ethylene-acrylic acid (EAA) copolymers and ethylene vinyl acetate (EVA) copolymers.
"Draw resonance" or "melt surging" occurs in LLDPE, ULDPE and other linear polymers such as high density polyethylene (HDPE), polypropylene and polyester during processing that involves rapid drawing or pulling of the melt such as extrusion coating, extrusion cast film fabrication, profile extrusion and fine denier fiber spinning. Also, the onset or occurrence of draw resonance is unmistakable.
The patent teachings of Kurtz et al. in U.S. Pat. No. 4,339,507 and Lucchesi et al. in U.S. Pat. No. 4,486,377 (the disclosures of both of which are incorporated herein by reference) describe draw resonance as a sustained random and/or periodic oscillation, variation or pulsation of the polymer melt with respect to the velocity and cross-sectional area of a melt drawing process that occurs between the die and the take-off position when the boundary conditions are a fixed velocity at the die and a fixed velocity at the take-off position. Draw resonance occurs when the draw ratio (i.e., the melt velocity at take-off divided by the melt velocity instantaneous at the die exit often approximated by dividing the reciprocal of the final polymer thickness by the reciprocal of the thickness of the melt instantaneous at the die exit) exceeds a polymer specific critical value. Draw resonance is a melt flow instability that is manifested as irregularities in the final coating, film or fiber dimensions and often produce widely variable thicknesses and widths. When line speeds significantly exceed the speed of onset, draw resonance can cause web or filament breaks and thereby shut down the entire drawing or converting process.
Given the various differences and intricacies that can exist between different extrusion equipment, relative resistance to draw resonance is often expressed in terms of critical draw ratio, and for conventional linear ethylene polymers, maximum stable draw ratios have been found to be less than 10:1, although draw ratios greater than 20:1 are needed for most commercial drawing operations.
"Drawdown" is defined herein to mean stretching or elongating a molten polymer extrudate (web or filament) in the machine direction and occasionally (simultaneously to a lesser degree) also in the transverse direction.
"Melt tension" which is also referred to in the relevant art as "melt strength" is defined and quantified herein to mean the stress or force (as applied by a wind-up drum equipped with a strain cell) required to draw a molten extrudate at some specified rate above its melting point as it passes through the die of a standard plastometer such as the one described in ASTM D1238-E. Melt tension values, which are reported herein in grams, are determined at 190 C. using a 2.16 kg weight and a 30 rpm wind-up rate. In general, for conventional linear and high pressure ethylene polymers, melt tension tends to increase with increased molecular weight, or with broadening of the molecular weight distribution and/or with increased melt flow ratios.
"Neck-in" which is influenced by extrudate swelling and, to lesser degree, by surface tension effects is defined herein as the difference between the die width and the web width at the taken-off position. Measured neck-in values (at constant output) will remain constant or decrease as the drawdown rate increases, and, in general, it is well known that for conventional ethylene polymers neck-in values increase as molecular weight decreases and/or as the molecular weight distribution narrows.
"Take-off position" is defined herein to mean the contact point (either the top or bottom) of a roller device that draws or pulls the molten extrudate down from its initial thickness instantaneous at the die exit to its final thickness. The roller device can be a nip roll, rubber roll, a chill roll, combinations thereof, or the like constructed from, for example, metal or rubber with various surfaces such as polished, matte or embossed finishes; all of which can to varying degrees affect the onset of draw resonance.
A variety of potential solutions have been disclosed to address the neck-in and/or draw resonance tendencies of linear polymers. Many of these solutions are equipment related and others primarily relate to modification of the properties of the linear polymers by forming polymer blends with highly branched high pressure ethylene polymers such as, for example, low density polyethylene. Thompson in U.S. Pat. No. 4,348,346 (which is incorporated herein by reference) is an example of equipment related attempts to address neck-in and draw resonance. Thompson describes a secondary injection of polymer melt streams into the primary die at the edges of the primary web stream is described to reduce neck-in and provide improved edge bead control.
An equipment modification solution specific to retarding the onset of draw resonance is provided by Cancio et al. in U.S. Pat. No. 4,668,463 and U.S. Pat. No. 4,626,574 (the disclosures of both of which are incorporated herein by reference) where locating a draw roller not more than 6 inches (15.2 cm) from the die provides a short air/draw gap and reduced draw resonance. Luchessi et al. in U.S. Pat. No. 4,486,377, teaches the use of a fluid medium, e.g., nitrogen, carbon monoxide or air, directed against the molten web prior to the take-off position as a viable method of retarding draw resonance. Similarly, Kurtz et al. in U.S. Pat. No. 4,608,221 (the disclosure of which is incorporated herein by reference) discloses that draw resonance can be mitigated by the utilization of a tensioning device with a friction free surface in a "rapid cooling zone" between the die and the take-off position.
Conversely, as another equipment modification example for alleviating or reducing draw resonance, Chaing in U.S. Pat. No. 4,859,379 (the disclosure of which is incorporated herein by reference) discloses radiant heating of the molten web prior to a chill roll take-off position.
Examples of modified linear polymer compositions exhibiting reduced draw resonance include U.S. Pat. No. 4,378,451 (Edwards), the disclosure of which is incorporated herein by reference, which discloses high flow rate compositions based on degraded propylene polymers blended with low density polyethylene. A similar example is provided by Werkman et al. in U.S. Pat. No. 3,247,290 (the disclosure of which is incorporated herein by reference) wherein thermally degraded (visbroken) high density polyethylene is blended with low density polyethylene to prepare high drawdown extrusion coating compositions. Another linear polymer blend example involving low density polyethylene is disclosed by Kurtz et al. in U.S. Pat. No. 4,339,507 where high pressure LDPE at 20 to 98 weight percent in combination with heterogeneous conventional LLDPE is taught to provide extrusion coating compositions with improved running rates.
An example of compositions that reduce draw resonance without the inclusion of a polymer degradation step and/or blending with a branched high pressure ethylene polymer is taught by Dohrer et al. in U.S. Pat. No. 4,780,264 where LLDPE with melt flow ratios less than 8.3 (i.e., utilizing molecular weight distributions even more narrow than typically employed) were found to allow surprisingly fast line speeds in extrusion coating and extrusion casting. However, predictably, these materials also exhibit higher neck-in and/or poor extrusion processability (e.g., higher extruder amperage).
In spite of the various advances, there is still a need for avoiding draw resonance and high neck-in problems when extruding known linear olefin polymer compositions at high extrusion line speeds. More particularly, there is also a need for extrusion coating and extrusion-casting materials that allow even higher line speeds and better extrusion processability than the lower melt flow ratio linear olefin polymers taught by Dohrer et al. in U.S. Pat. No. 4,780,264 and the polymer blends described by Kurtz et al. in U.S. Pat. No. 4,339,507. It is also desirable to avoid mechanical or equipment modifications that are designed to alter the drawing process itself as a means of attaining greater resistance draw resonance and lower neck-in. Additionally, to a lesser extent, there is also a desire to avoid the extra detail involved in providing appropriately degraded polymers as separately described by Werkman et al. and Edwards. As described hereinafter, the present invention substantially fills such desires and needs by providing compositions comprising at least one substantially linear ethylene polymer. However, if cast film or extrusion coating equipment modifications are already installed, and/or if blending with thermally degraded polymers is desired, or if the use of linear olefin polymers with lower melt flow ratios is desired, the present invention can still be employed to good advantage and the combined or synergistic benefits of the present invention and known solutions can also be realized.